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High-performance silicon photoanode using nickel/ iron as catalyst for efficient ethanol oxidation reaction Qian Cai, Wenting Hong, Chuanyong Jian, Jing Li, and Wei Liu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b04661 • Publication Date (Web): 23 Jan 2018 Downloaded from http://pubs.acs.org on January 23, 2018

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ACS Sustainable Chemistry & Engineering

High-performance

silicon

photoanode

using

nickel/iron as catalyst for efficient ethanol oxidation reaction Qian Cai, † Wenting Hong, †‡ Chuanyong Jian, † Jing Li, † and Wei Liu*† † CAS Key Laboratory of Design and Assembly of Functional Nanostructures, and Fujian Provincial Key Laboratory of Nanomaterials, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, No. 155, Yangqiao West Road, Fuzhou, Fujian, 350002, China ‡ University of Chinese Academy of Sciences, Beijing, 100049, China * Corresponding authors: Wei Liu, E-mail: [email protected] KEYWORDS: silicon photoanode, ethanol oxidation reaction, solar energy, electrocatalysis

ABSTRACT: In this study, we report NiFe/ZrO2/n-Si photoanodes with efficient photoelectrocatalytic activity for ethanol oxidation reaction (EOR) in alkaline medium (1.0 M KOH). NiFe/ZrO2/n-Si electrode exhibits apparently improved EOR activity with the massspecific activity of 34.4 mA·cm-2 under visible light illumination, which is approximately 2.53 times higher than that in the dark (13.6 mA·cm-2). NiFe/ZrO2/n-Si electrode presents obviously enhanced stability under visible light illumination due to the acceleration of the oxidation of

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intermediate products adsorbed on the surface of the catalyst. Additionally, NiFe/ZrO2/n-Si photoanode displays better EOR activity compared with Pt(Pd)/ZrO2/n-Si photoanodes, which shows the potential to replace the Pt or Pd in EOR using silicon photoanode.

Introduction The photoelectrochemical (PEC) processes have been widely used to drive water oxidation reaction to provide electrons for hydrogen production in water electrolysis. However, oxygen evolution reaction (OER) process is kinetically sluggish with large overpotential due to the multistep four-electron oxidation process.1-4 Recently, the oxidation of small organic molecular, such as ethanol instead of water, has been realized as a promising route to provide electron for hydrogen production to overcome the kinetic limitations of water oxidation.5 In PEC system, the integration of electrocatalyst with semiconductors such as WO3, TiO2, ZnO, ZnS is considered as an effective strategy to boost EOR performance with the assistance of solar energy.6-8 The semiconductor can generate photoelectron-hole pairs which may further participate in redox reactions, therefore enhancing the EOR electrocatalytic performance. To efficiently utilize solar energy and promote the EOR performance, semiconductors with narrow bandgap which have a strong ability to absorb visible light need to be further explored. 9-12 Semiconductors with small band gap, such as silicon (Si) with a small band gap (~1.1 eV) allowing absorbing a large portion of the solar spectrum, has attracted great interests in photovoltaic and photoelectrochemical fields.13-15 To avoid corrosion, Si is integrated into a metal-insulator-semiconductor (MIS) device geometry, which is not only beneficial for inhibiting Si corrosion but also promoting the electron-hole separation efficiency due to the confined electron and hole transport directions under bias (Figure 1a). However, the application


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of Si MIS photoanode in ethanol oxidation has rarely been studied. In our previous work, we firstly successfully employ Si MIS photoanode using Pt and Pd electrocatalysts for ethanol oxidation, which exhibits apparently improved ethanol oxidation reaction (EOR) activity and durability with the assistance of visible light.16 Platinum-based (Pt) catalyst has been proved to be the most efficient electrocatalyst for EOR. However, their practical and broad application in EOR is heavily hampered by its susceptibility to poisonous intermediate species and the high cost.17-20 Various low-cost non-noble metals with comparable catalytic activity have been investigated as the alternate candidates. Nickel-based metal catalysts have attracted great attention owing to their reasonable catalytic activity and low cost. Nickel oxide formed in EOR process provides an oxygen source which can oxidize the adsorbed CO-like intermediates at low potentials.21-23 However, the activity and stability of Ni-based EOR electrocatalysts are still not satisfactory compared with Pt. Numerous study indicates that introducing a second metal such as the transition metals Co, Cu or Fe is an effective approach to promote the EOR efficiency for Ni-based electrocatalysts.24-26 The charge transfer between the binary atoms will lead to a weakened CO-like intermediate adsorbed on the electrode. In our previous work, Fe was employed to alloy with Ni as a binary catalyst for water oxidation reaction, which exhibits an excellent catalytic performance.27 Herein, we report a high-performance NiFe/ZrO2/n-Si electrode for efficient EOR. The NiFe/ZrO2/n-Si electrode shows the higher performance of EOR activity compared with Pt(Pd)/ZrO2/n-Si. Under the assistance of visible light, both the catalytic activity and the stability of silicon photoanode are remarkably improved. The current density of NiFe/ZrO2/n-Si electrode is up to 34.4 mA·cm-2, which is approximately 2.53 times higher than that in the dark (13.6


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mA·cm-2). In addition, NiFe/ZrO2/n-Si electrode retains its EOR activity around 95.1 % after 4000 CVs scans, indicating high stability. Results and discussion Morphological and structural of NiFe/ZrO2/n-Si Figure 1a schematically illustrates the energy schematic diagram of NiFe/ZrO2/n-Si electrodes for ethanol oxidation. The ultrathin ZrO2 layer serves as the protective layers to prevent Si corrosion as well as the electron filter layer for photo-holes transport. The ethanol molecules can be oxidized by the holes on the NiFe surface. To examine the device structure of as-prepared NiFe/ZrO2/n-Si electrode, the cross-sectional image of NiFe/ZrO2/n-Si photoanode is obtained by HRTEM as shown in Figure 1b. It reveals that Si photoanode has a good interface without apparent defects and cracks. The layer thickness of ZrO2 and NiFe are confirmed to be 2.3 nm and 2.2 nm, respectively, which is consistent with the deposition thickness as described in the fabrication process. Scanning transmission-electron microscopy-energy-dispersive spectroscopy (STEM-EDS) mapping of NiFe/ZrO2/n-Si interface displayed in Figure 1c further depicts that the Si photoanode has relatively well-defined layers of Si, ZrO2, and NiFe. The optical absorption measurement of Si photoanode shows that the ultra-thin NiFe layer will effectively be optically “transparent” and not significantly affect the light absorption of Si substrate (Supplementary information Figure S1). AFM image of NiFe/ZrO2/n-Si surface presented in Figure 1d exhibits a granular morphology with the root-mean-squared (r.m.s) surface roughness of 1.55 nm, which demonstrated that the deposited 2 nm NiFe will not form a continuous film. To further confirm the surface-active component of NiFe/ZrO2/n-Si electrode in EOR process, XPS measurements were performed on the surface of silicon photoanode of pristine and silicon photoanode after EOR testing, respectively. The survey XPS spectra shown in Figure 2a


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indicates that the overall composition is mainly composed of Ni, Fe, C, Zr, O and Si elements for silicon photoanode. As shown in high-resolution Ni 2p XPS spectra (Figure 2b), it is obvious that the peak of metallic Ni (∼852.8 eV) vanished after OER testing, confirming that an entire surface Ni oxidation occurred during the electrochemistry process.

Figure 1. (a) Band diagram of NiFe/ZrO2/n-Si photoanode for ethanol oxidation reaction. (b) High-resolution transmission electron microscope (HRTEM) image of a cross-section of the NiFe/ZrO2/n-Si photoanode. (c) Energy-dispersive spectroscopy (EDS) mapping-scan across the NiFe/ZrO2/n-Si interface. (d) AFM image of the surface of NiFe/ZrO2/n-Si photoanode. The peak at 855.8 eV existed in both samples exhibit the coexisting valence states of Ni2+ (2p3/2, ~857.1 eV) and Ni3+ (2p3/2, ~855.5 eV), which can be assigned to NiOOH or Ni(OH)2 phase.28 The Fe 2p spectra present a similar change tendency with Ni after the EOR operation as shown in Figure 2c. The vanished metallic Fe at 706.2 eV demonstrated that the oxidation


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reaction occurred at the surface of silicon photoanode in EOR process. Peaks at 711.3 eV and 714.6 eV for Fe3+ can be attributed to the FeOOH and Fe2O3.29 It is known that NiOOH/Ni(OH)2 or FeOOH/Fe2O3 are the active components responsible for EOR. The changes of Ni and Fe can also be reflected by the O1s XPS spectra (Figure 2d). It reveals that three peaks corresponding to classical oxygen atoms in three different environments include lattice oxygen (530.1 eV), dissociative adsorbed water (OH) (531.5 eV), and surface-absorbed water (532.8 eV).30

Figure 2. (a), Survey XPS spectra of NiFe/ZrO2/n-Si electrode before and after EOR operation. High-resolution XPS spectra of Ni 2p spectra (b), Fe 2p spectra (c) and O 1s spectra (d) before and after EOR operation. Ethanol oxidation catalytic performance The

electro-oxidation

performance

of

as-prepared

Ni/ZrO2/n-Si,

Fe/ZrO2/n-Si

and

NiFe/ZrO2/n-Si electrodes are evaluated by Cyclic voltammograms (CVs) curves in 1.0 M KOH


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electrolyte at a sweep rate of 50 mV·s-1 in the dark as shown in Figure 3a. The CVs curves of Ni/ZrO2/n-Si (blue line) and NiFe/ZrO2/n-Si (red line) electrodes depict a characteristic redox peak associated with the oxidation/reduction of Ni (at 0.25 V, Figure 3a). NiFe/ZrO2/n-Si electrode

displays

a

stronger

electrochemical

response

with

narrower

and

higher

oxidation/reduction current compared with Ni/ZrO2/n-Si. Fe/ZrO2/n-Si electrode (black line) shows obviously different redox peaks at more positive potential attributed to the Fe oxidation. When 1.0 M ethanol is added into 1.0 M KOH solution, the CVs curves of silicon photoanodes are dramatically changed as shown in Figure 3b. In all cases, a distinct anodic peak appears at ~0.16 V in the forward scan is attributed to the oxidation of ethanol. The oxidation current reaches a maximum value and then decreases, which can be attributed to the formation of an inactive compact oxide layer on the electrode surface, thereby inhibiting the ethanol adsorption on the catalyst surface. In reverse scan, the oxidation continued, and its corresponding current go through a maximum value at the potential of 0.07 V owing to the regeneration of active adsorption sites for ethanol by incompletely removing the oxidized carbonate intermediates. The influence of different weight ratio of Ni/Fe on the EOR performance also has been investigated (Figure 3b). NiFe/ZrO2/n-Si electrode with Ni/Fe weight ratio of 80/20 exhibits the largest EOR specific current density (13.6 mA·cm-2) among all electrodes, which is approximately 1.87 times and 6.24 times higher than that of Ni/ZrO2/n-Si electrode (7.28 mA·cm-2) and Fe/ZrO2/n-Si electrode (2.18 mA·cm-2), respectively. (NiFe represents the weight ratio of 80/20, except where otherwise noted.) The remarkably enhanced EOR activity for NiFe/ZrO2/n-Si electrode signifies that the proper Ni/Fe weight ratio can significantly improve the electrochemical activity. The improved EOR activity for Fe doping can be presumably attributed to the following aspects: (1) Fe doping may result in partial-charge transfer from Fe sites to activate Ni centers, and enhance


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the catalytic activity; (2) doping with Fe is contributing to an improved conductivity, beneficial in increasing the reaction efficiency; (3) Fe3+ containing compounds such as Fe(OH)3 and FeOOH in a strong alkaline solution tend to encourage the formation of OH adsorption (OHads) species, which are responsible for improving the poison tolerance.8,31,32 Recent studies have shown that ethanol concentration (Cethanol) is a crucial factor for EOR.33 The CVs curves of NiFe/ZrO2/n-Si electrode in 1.0 M KOH electrolyte with different ethanol concentration are displayed in Figure 3c. The ethanol oxidation peak current is proportional to the ethanol concentration. The increase of ethanol concentration can cause a nearly comparable enhancement of the anodic peak current density (Figure 3d). This linear relationship between anodic current density and ethanol concentration indicates that the ethanol oxidation process is controlled by the diffusion of ethanol on the surface of the catalyst. Moreover, this diffusion controlled process is further verified by the impact of scan rates on the EOR activity for NiFe/ZrO2/n-Si electrode as shown in Supplementary Figure S2. The peak current density is also linearly proportional to the square root of scan rates, which is the characteristic of a diffusion controlled process limited by the diffusion of ethanol or the reaction intermediates.8,34 CVs curves for NiFe/ZrO2/n-Si photoanode in the electrolyte of 1.0 M KOH and with and without 1.0 M ethanol under simulated 1 sun (100 mW·cm2, 1.5 AM) illumination from the front side are presented in Figure 3e. In the absence of ethanol, NiFe/ZrO2/n-Si electrode presents excellent oxygen evolution reaction (OER) activity with the onset potential at 0.15 V versus Ag/AgCl electrode under the light illumination (solid blue line), while the electrode in the dark (dotted blue line) does not exhibit any water oxidation catalytic activity. When the NiFe/ZrO2/nSi photoanode is measured in 1.0 M ethanol, both the obvious OER and EOR processes can be observed from CV curves (solid green line). To exclude the partial photocurrents derived from


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OER, the EOR photocurrent (red solid line) can be obtained through the background subtraction, while the OER process (blue solid line) is regarded as the background.

Figure 3. (a) CVs curves of Si photoanodes with Ni, Fe, and NiFe catalyst measure in the dark. (b) CVs curves of NiFe/ZrO2/n-Si in the dark with different NiFe weight ratios (Ni/Fe=100/0, 80/20, 60/40, 40/60, 0/100). (c) CVs curves of NiFe/ZrO2/n-Si measured in 1.0 M KOH solution with different ethanol concentrations in the dark. (d) Oxidation peak current densities versus ethanol concentration of Si photoanode in the dark. (e) CVs curves of NiFe (2 nm)/ZrO2/n-Si electrode measured in 1.0 M ethanol+1.0 M KOH electrolyte with and without visible light illumination (100 mW·cm-2). (f) CVs curves of NiFe (4 nm)/ZrO2/n-Si electrode measure in 1.0 M KOH and 1.0 M ethanol+1.0 M KOH electrolyte with and without visible light illumination (100 mW·cm-2). (g) Relationship between the oxidation peak current densities and the incident light intensity (60, 80, 100, 120, and 140 mW·cm-2). (h) CVs curves of Pt/ZrO2/n-Si, Pd/ZrO2/nSi and NiFe/ZrO2/n-Si measured under light illumination (100 mW·cm-2). (i) The histogram of


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EOR oxidation current density for various Si photoanodes with and without visible light illumination. From background subtraction curve (solid red line, Figure 3e), the onset potential requires for EOR under light illumination is only -0.11 V versus Ag/AgCl electrode, which is approximately 260 mV less than that for water oxidation, indicating that EOR requires lower overpotential, therefore reducing the usage of the energy compared with water oxidation. The ethanol oxidation current density is another significant factor in determining the catalytic activity of EOR electrode. The ethanol oxidation current is significantly improved compared with that in the dark. The NiFe/ZrO2/n-Si electrode in the presence of light illumination shows high oxidation peak current density of 34.4 mA·cm-2 (solid red line, Figure 3e), which is approximately 2.53 times greater than that in the dark (13.6 mA·cm-2, red dotted line, Figure 3e). In addition, the CVs curves of Si photoanode with light illumination exhibit two successive current oxidation peaks (marked with brown box, Figure 3e) in the reverse scan which is a significant difference with the CVs curves measured in the dark (dotted red line). The peak in the low potential (box1, Figure 3d) is consistent with the oxidation peak in the dark, which is attributed to the removal of incompletely oxidized carbonate intermediates adsorbed on the catalyst surface. The new peak at high potential region (box 2, Figure 3e) also can be assigned to the intermediate oxidation in the EOR process.35 It is considered that the presence of visible light will facilitate the oxidation of intermediate products adsorbed on the catalyst surface and regeneration of active adsorption sites for ethanol, thereby promoting the poison tolerance of catalyst and benefitting the electrochemical oxidation of ethanol in strong alkaline solution. Additionally, the influence of the thickness of NiFe layer and light intensity on EOR performance was further investigated. It is well known that metal layer plays a key role in Sibased MIS photoanodes.14 The metal layer should not only thick enough to provide enough


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catalyst, but also thin enough not blocking the transmission of photons into the Si substrate.27 4 nm NiFe layer also fabricated and employed for EOR as shown in Figure 3f. In the absence of light irradiation, the NiFe (4 nm)/ZrO2/n-Si electrode exhibits a similar EOR activity with NiFe (2 nm)/ZrO2/n-Si electrode. However, the NiFe (4 nm)/ZrO2/n-Si electrode shows weak photoresponse compare with NiFe (2 nm)/ZrO2/n-Si electrode, which can be ascribed to the thick NiFe layer significantly affect the light absorption of the Si substrate. Hence, 2 nm NiFe layer was considered as the proper thickness for Si-based photoanodes which can provide effective catalytic ability and excellent light penetration capacity. Figure 3g and Figure S3 present the relationship between the EOR performance and the incident illumination intensity. The results indicate that the photocurrent both on OER and EOR process increased as the light intensity increases, while the EOR peak current density is nearly linearly with light intensity.15 Generally, 100 mW·cm-2 (1.5 AM) signifies the overall yearly average for mid-latitudes, which is the standard incident light intensity for all standardized testing or rating of terrestrial solar cells or modules. Hence, 100 mW·cm-2 was selected as the light intensity for EOR process in this work. Based on electrochemical detection and analysis, both the negative shift of onset potential and enhancement of photocurrent indicate that the NiFe/ZrO2/n-Si electrode owns excellent EOR catalytic activity under visible light illumination. Interestingly, it is found that for this Si-based MIS electrode system, the low-cost NiFe metal catalyst deposited onto ZrO2/n-Si substrate under visible light illumination exhibits the unusual superior EOR activity compare with the conventional active precious metal catalyst such as Pt or Pd. As shown in Figure 3h and Figure S5, the oxidation current density of NiFe/ZrO2/n-Si electrode is, of course, higher than Pt/ZrO2/n-Si and Pd/ZrO2/n-Si electrodes both in the dark and under light irradiation. Certainly, the particular reason is still needed to be further explored in the future works. Nevertheless, this


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result provides the great potential to reduce the cost of high efficient EOR process. In addition, the specific activity for all the electrodes is compared and extracted in Figure 3i. It is obviously observed that the specific activities for all the electrodes are remarkably enhanced under visible light illumination. The comparison results imply that the visible light illumination efficiently improves the EOR activity which could be attributed to the synergistic effects of photo- and electrocatalysis of ethanol oxidation.

Figure 4. (a), the transient photocurrent (TPC) spectra for Ni/ZrO2/n-Si, Fe/ZrO2/n-Si and NiFe/ZrO2/n-Si electrodes collected at the EOR oxidation potential. (b), EIS spectra for the three different electrodes under visible light illumination. The inset shows the equivalent circuit model. (c), the durability comparison of CVs curves for NiFe/ZrO2/n-Si electrode with and without visible light illumination. (d), the normalized peak current density versus CVs cycles.


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The photocurrent response for the EOR is recorded by transient photocurrent (TPC) measurements under the chopped light illumination (Figure 4a). It is observed that all the three electrodes show excellent photo-response feature, while the NiFe/ZrO2/n-Si electrode exhibits favorable steady-state behavior with faster signal rises and decays without the appearance of current spikes, further confirming the effective charge separation and the reduced charge recombination. The improved charge transport and recombination properties of NiFe/ZrO2/n-Si electrode also can be confirmed by electrochemical impedance spectroscopy (EIS) spectra as shown in Figure 4b. EIS measurements are conducted with the frequency ranging from 1 Hz to 10000 Hz to explore the charge transport and recombination properties of the photoelectrodes. The fitted EIS spectra (Figure 4b) under light illumination for all electrodes reveal two semicircles, which can be modeled to an equivalent circuit model (Figure 4b inset). The first and the second semicircle are ascribed to the bulk and surface charge transfer processes, respectively. The bulk charge transfers resistance (Rbulk) is derived from the underlying Si substrate under light illumination, 36 while the second semicircle depicts the charge transfer resistance from the surface catalyst to the electrolyte. The results indicate that NiFe/ZrO2/n-Si electrode has the smallest surface charge transfer resistance, leading to the efficient separation of photogenerated electron-hole pairs, and hence the fast-interfacial charge transfer, which is consistent with the best EOR performance. The EIS spectra of different Si photoanodes in the dark are shown in Figure S6, while the NiFe/ZrO2/n-Si electrode similarly has the smallest surface charge transfer resistance compare with other electrodes. Indeed, Fe doping plays a significant role in influencing the EOR performance by cause partial-charge to transfer from Fe sites to activate Ni centers and improve the conductivity, eventually enhancing the EOR catalytic activity. To better investigate the catalytic oxidation performance of NiFe/ZrO2/n-Si electrode, another commonly


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small fuel molecule, methanol, is also used as a probe molecule. From the CVs curves and TPC spectrum shown in Figure S7, the NiFe/ZrO2/n-Si electrode shows improvement methanol oxidation ability under visible light illumination. Besides, it presents remarkable photo-response feature. Long-term stability is another important parameter for the application of electrodes in fuel cells. To evaluate the EOR durability, NiFe/ZrO2/n-Si anode is continually scanned in 1.0 M KOH electrolyte containing 1.0 M ethanol for 4000 CVs cycles both in the dark and under visible light illumination. The comparison of CVs curves and peak current density versus CVs cycle numbers are shown in Figure 4c and 4d, respectively. After 4000 CVs cycles, NiFe/ZrO2/n-Si electrode retain the EOR activity around 95.1 % and 58.3 % with and without light illumination, respectively. This result indicates that the visible light can dramatically promote the electrocatalytic stability of NiFe/ZrO2/n-Si electrode. The light will accelerate the oxidation of intermediate products adsorbed on the electrode surface and improve the anti-poison property, thereby enhance the electrode stability in continuous EOR operation in strong alkaline solution.


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Scheme 1. Schematic of a possible mechanism for photo-assisted ethanol oxidation process for NiFe/ZrO2/n-Si electrode under visible light illumination. The remarkably improved EOR activity and stability for NiFe/ZrO2/n-Si electrode under visible light illumination can be attributed to the synergistic effects of electro- and photocatalytic during the EOR catalytic processes as shown in schematic 1. The ethanol electro-oxidation on NiFe catalyst and the photoelectron-oxidation on Si simultaneously occurred in the whole process. When the electrode exposed to the visible light illumination, Si will absorb the light and stimulate the electrons (e-) jump from the valence band (VB) to the conduction band (CB), leaving photo-generated holes (h+) in the conduction band. The thin ZrO2 layer deposited on Si substrate serves as the electron filter, which allows for photo-generated holes tunneling through ZrO2 layer to the surface of NiFe catalyst. Then, the holes can react with surface adsorbed OH−/H2O to form high oxidative hydroxyl radicals (•OH) (process 2). The •OH will further oxidize the ethanol resulting in photo-assisted EOR at the anode (process 3).37,38 Additionally, the highly reactive •OH can further oxidize the common carbonaceous species like COads and carbonaceous species absorbed on the surface of catalysts (process 4) generated both from electro-oxidation and photoelectron-oxidation process, which will quickly removal nonreactive products and regenerate the active sites on NiFe surface for further ethanol oxidation. On the other hand, the photoelectrons were transported to the cathode through an external circuit, wherein oxygen was efficiently reduced (process 5). Therefore, the catalytic activity and stability of ethanol oxidation can be both improved effectively with the assistance of visible light illumination. Conclusion


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In summary, we report a high-performance NiFe/ZrO2/n-Si photoanode for EOR. NiFe/ZrO2/nSi photoanode can promote the EOR catalytic activity and stability which can be attributed to the synergistic effects of electro- and photocatalytic during the EOR processes. NiFe/ZrO2/n-Si electrode exhibits higher EOR activity compared with Pt(Pd)/ZrO2/n-Si electrodes, which shows the potential to replace the Pt or Pd catalysts in EOR using silicon photoanode. When exposed to the visible light illumination, The NiFe/ZrO2/n-Si electrode exhibits apparently improved EOR activity with the specific activity of 34.4 mA·cm-2 and enhanced stability. This work provides a simple approach to fabricate high-performance and low-cost Si-based EOR electrodes, which has enormous potential for application in direct ethanol fuel cells and hydrogen production.

Supporting Information. Detailed descriptions of the experimental and characterization methods, UV-Vis spectra, Cyclic voltammograms (CVs) curves at different scan rates, EIS spectra in the dark, CV curves for NiFe/ZrO2/n-Si electrode under various light intensity, Table of EOR and MOR catalytic parameters of recently-reported photo-assisted electrodes are presented in Supporting Information. AUTHOR INFORMATION Corresponding Author * [email protected] Notes The authors declare no competing financial interests. ACKNOWLEDGMENT


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This work was supported by Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, the National Natural Science Foundation of China (No. 61674152), the Natural Science Foundation of Fujian Province of China (No. 2017J01130).

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TOC

The NiFe/ZrO2/n-Si electrode with efficient electrocatalytic and photoelectrocatalytic activity for EOR are fabricated, which shows the potential to replace the Pt or Pd catalysts in EOR using silicon photoanode.


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iron as catalyst for

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